Electro-optical image production



March 3, 1942. l w. A, KNooP I ELECTRO-OPTICAL IMAGE PRODUCTION Filed April 28,1939 4 sheet-sheet 2 HUN@ /Nl/ENTOR WA. KNOOP A T TORNE V March 3, 1942. w. A. KNooP ELECTRO"OPTICAL `IMAGE PRODUC'I'ION Filed April 28, 1959 4 -sheets-'sheet s- `V HHIHU /Nl/ENTOR WAKNOOP ATTORNEY w. A. KNQOP 2,274,710

ELECTRO-OPTICAL IMAGE yPRODUCTION l Filedv April 28, 1939 4 Sheets-Sheet 4 March 3, 1942.

Patented Mar. 3,' 1942 ELECTRO-OPTICAL IMAGE PRODUCTION William A. Knoop, Hempstead, N. Y., assignor-to Bell Telephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New York Application April 28, 1939, Serial No. 270,476

5 Claims.

other amplitude variations produced as the result of periodically reducing the cinrent to a value equal to or preferably less than that produced when scanning an area of black tone value. Such a reduction in amplitude may be introduced at the end of each scanning line of a ield of view, as, for example, by periodically interrupting the scanning of the field of view and, therefore, the light activation of the light sensitive element employed -in scanning. The electrical transmitting apparatus upon which this unidirectional image current is impressed suppresses the direct component which is representative of the average tone value of the ileld being scanned and 'which varies in amplitude as the average tone value vof the ileld changes. The

amplitude of the steady component of the univ directional image current, which component is representative of the average tone value over a given period, varies in accordance with the amplitude of the impulses set up as the result of periodically interrupting the scanning. In the alternating image current, the amplitude of the impulses produced as the result of periodically interrupting the scanning, measured between the peaks of the impulses and zero, varies in accordance with the amplitude of theV suppressed direct component and, therefore, in accordance with the average tone' value of the elemental areas scanned between successive interruptions of the scanning. These black or correcting impulses are utilized at the transmitter or receiver for controlling apparatus for correcting for the suppression of the direct component of the unidirectional image current.

In accordance with a preferred embodiment of the invention, the alternating image current or electromotive force including the correcting impulses is' impressed upon a device which re- .ponds only to the correcting impulses to control a source of current, and current from this source is utilized in correcting for the suppressionof the direct component ofthe image current. The

average amplitude of the current from this source varies in accordance with the amplitude of the correcting impulses. The correcting de- `vice includes one or more gas-nlled tubes, commonly known as thyratrons. These tubes have the characteristic of passing current in the discharge or plate circuit when the potential on the grid is sufiiciently high, for a given voltage across the plate circuit and thereafter continuing to pass current in the plate circuit independently of subsequent changes in the grid potential until the plate voltage is reduced below the ionization potential. An alternating potential of a frequency which is low compared with the frequency of recurrence of the correcting impulses is applied to the grid and anode circuits of the-gas-lled tube or tubes, the voltage applied to the grid being approximately 180 degrees out of phase with respect to the potential applied to the anode. The correcting impulses are superposed upon the voltage applied to the grid circuit.

The anode current of the gas-filled tube after being ltered to suppress the alternating c omponents flows through an output resistor across which is connected a small condenser, the current through this output resistor being a slowly varying unidirectional current. The condenser which becomes charged to a voltage corresponding `to the direct component representative of average image tonevalue is connected in the control circuit of a cathode ray image producing tube for biasing it, the television signal also being impressed upon the control circuit of the cathode ray tube.

In another embodiment of the invention, the anode current of the gas-lled tube ows through a biasing winding of each of two impulse coils of the` type disclosed in United States Patent 1,880,412 to Burton, October 4, 1932. These impulse coils have cores which are constricted in cross-section inthe portions where the secondary winding is located. This portion of the core thus saturates quickly and for this reason a 7 sharp voltage impulse of short duration and high amplitude is produced in the secondary winding every time the magnetic field in the constricted monly known as thyr'ite, for example, may be included in the circuit in series with the light producing device, a gaseous discharge lamp, for example. in order to obtain a straight line relationship between the current Aflowing through the lamp and the thyrite element in series and the electromotive force across the lamp and the thyrite element. also be included in series with the lamp and thyrite element to help straighten out the characteristic.

Thelinvention will be more readily understood from the following description taken in connection with the accompanying drawings in which:

Fig. 1 is a diagrammatic view of a television system including apparatus controlledy .by a

-source of image current having only alternating components for producing a slowly varying unidirectional electromotive force for use in controlling the average tone value of the reproduced' image;

Fig. 2 is a detail showing of a portion of the scanning apparatus of Fig. 1;

Figs. 3 and 4 are diagrammatic views of modifications of a portion of the television system of Fig. 1; and

Figs. 5 to 9, inclusive, are curves which will bc i used in connection with Figs. 1 to 4 to explain vsource to pass through the successive apertures 22 to thereby produce a moving beam of light which is focussed by the lens system 24 to illuminate elemental areas of the eld of View 0 in succession. The disc 2| is rotated at a speed such that, for one complete revolution, elemental areas of parallel adjacent lines of the complete field of view 0 are successively illuminated within the period of persistence of vision.. Light reflected from the successive elemental areas of the field of' view li reaches a light sensitive electric device 25, which is thereby activated to produce an image current varying in accordance with the tone values of such elemental areas. In front of the disc 2| is a screen 26 of opaque material provided with an opening 21- which determines the size of the field 0. The opening 21 is so dimensioned that light passes through the apertures 22, one at a time, to the eld of view 0.

For the purpose of the present invention, the scanning apparatus operates in such a manner that the scanning of the field of view is periodically interrupted for a brief interval one or more Limes during cach complete scanning period. In

A small resistance element mayV a preferred arrangement for effecting this result, the lateral distance between the apertures 22 of the disc 2| is made greater than the width of the opening 21 in the screen 26, by an amount equal to the width of one or a few elemental\ areas, as shown in Fig. 2.

The light sensitive device 25 supplied with energizing current from the direct current source 28, will be activated, under control of the abovedescribed optical system, to produce a unidirectional image current. This unidirectional image current consists of a slowly varying direct component the amplitude of which corresponds to the averagel of the tone value or brightness of the field of view and alternating components of different frequencies, respectively, the ampli-- tudes of which are determined by the tone values of the successively scanned elemental areas of the field of view. The frequencies of these alternating components may extend from a frequency of a million cycles or more to a low frequency which is approximately the frequency of scanning of the entire field of view, i. e., around 20 cycles per second. When no light is applied to the light sensitive device, due' to the interruption of the scanning light beam, for example, the amplitude of the current flowing in the circuitv of the light sensitive device 25 will be reduced to a low value, substantially zero, for example.

While the major portion of the image current frequency band may be satisfactorily transmitted over well-known communication circuits including telephone lines or coaxial cables, for example, it is well known that such transmission circuits include apparatus which suppresses or greatly attenuates the direct and very low frequency components of a television signal.

The electromotive force across a resistive element 352 in circuit with the light sensitive device 25 is applied to a circuit including a coupling condenser 355 and the input circuit of anamplifier thermionic tube 29 which includes a high resistance 353 and a biasing battery 354. The

line L an alternating image current includingthe correcting impulses which are of larger amplitude than other portions of the image current and of a polarity the same as that produced by scanning black elemental areas.

At the receiving station the line L is connected through transformer |33 to a suitable amplifler |32 which may comprise a plurality of stages and the output circuit of amplifier |32 is connected through the amplifier tubes |34 and |35 to thev control electrode-cathode cirouit'of the cathode ray image producing device |36. The amplifier |32 is coupled to the amplifier stage |34 through the coupling condenser |31, grid resistor |38 and biasing battery |39. Battery |40 supplies anode current to vacuum tube |34 through resistor |4|. Vacuum tube |34 is coupled to vacuum tube |35 through coupling condenser |42, grid resistor' |43 and grid biasing battery |44. Battery |45 supplies anode current to vacuum tube |35 through resistor |46. vCurrent source |41 supplies current to the thermionic cathodes of vacuum tubes |34 and |35. The

` cathode ray tube |36 is of the type disclosed in cathode |41, a control electrode |68, an anode |59, defiectlng electrodes and |52 and a screen of fluorescent or phosphorescent material |53. The anode-cathode circuit of Yvacuum tube |35 is connected through coupling condenser |54 of 2 mlcrofarads to the control electrode |68 and through a condenser |55 of .02 microfarad -a portion of resistor |56 to the control electrodecathode circuit of the cathode ray tube |36. Sixty-cycle current from the source applied to motor 23 is also supplied to the harmonic generator |60 for producing 4,800-cyc1e current, which is supplied to the sweep control circuit 56|, the output of which is connectedto the line defiecting electrodes |5|. A submultiple frequency generator |62 controlled by current from the harmonic generator |66 supplies 20-cycle current to the low frequency sweep circuit |63, the output of which is connected to the frame defiecting electrodes |52. The cathode ray 'beam produced in the tube |36 thus scans the screen |66 in synchronlsm with the scanning of the image field at the transmitting station and the lbearn .is modulated in intensity under control of the alternating components of the television image current supplied to the cathode ray tube from the output ofthe amplifier tube |35, thus producing a television image of the view which is scanned at the transmitter.

The television image current transmitted to the receiver R does not include a slowly varying v direct component representative of the average tone value of the'fleld of view being scanned at the transmitting station T. However, the transmitted current has recurring impulses the amplitude of which measured between the peaks of the impulses and zero varies in accordance with the average tone value of the elemental areas scanned between the times that successive correcting impulses. are generated lat lthe transsensitive device associated with the transmitting` circuit and means for periodically directing light from the entire field upon the device.

For the purpose of controlling the production of images having an average tone value corresponding tothat of the view being scanned, there is provided a correcting circuit including vacuum tube amplifier |64 and gas-filled electric discharge device |65. The input circuitof vacuum tube |66 is connected in parallel with the input circuit of vacuum tube to the output of amplifier |32. Anode current is supplied to the vacuum tube |64 from battery 365 through resistor 366. The output of vacuum tube |64 is coupled to the control circuit ofthe gas-filled tube |65 through a circuit comprising condenser |66 of l0.2 microfarad, potentiometer |61 and grid-biasing battery |68, the condenser being of small capacity in order to transmit efficiently the fundamental frequency of the correctingy impulses and to attenuate low frequency com c ponents ofthe transmitted television signal. .A source of 1l0-volt, 60cycle alternating current |69 is connected to the anode circuit of tube |05 through transformer |10 and to the control electrode-cathode circuit through transformer |1|.

|56 is a direct current the amplitude of which is that of R being 15,000 ohms.

winding |1| are connected a condenser |12 of 2 microfarads and a resistor |13 of 2,500 ohms, one terminal of the resistor being connected to ground and a variable connection thereto being connected to the cathode of tube |65. A portion fof resistor |12 is ythus connected in the control may be provided, if desired) and, therefore, the

line marked EB in Fig. 5 should be zero when these curves are applied to the circuit shown in Fig. l. When the Ee curve becomes tangent to or intercepts the dash-line curve, gas-filled tube |65 breaks down and current flows in the anode circuit of the tube until the anode-cathode voltage is decreased below a certain value. If no correcting impulses were superposed upon the sine-wave voltage EG, the tube would break downI at point A during each cycle of the cycle voltage. When the correcting impulses are received and impressed upon the control circuitof tube |65, along with the 60cycle voltage, current will commence to flow sooner during each cycle of the 60cycle source, for example, at point B and the current will flow for a longer period. The larger the amplitude of the correcting impulse, the earlier during each cycle of the 60cycle source will tube commence tov pass space current and the larger will be the average amplitude of the anode current. The phase relationship of the 60cycle voltage applied to the anode and control circuits,'respectively, of tube` |65 is preferably such that the tube fires or commences to pass space current during a portion of the cycle in which the anode-cathode voltage is decreasing from maximum to zero and in.

which the control 'electrode-cathode voltage is increasing, the maximum amplitude of the correcting impulses being insufficient to cause the tube to fire" earlier in the cycle.

The output circuit of tube |65 is connected in `series with the variable resistive element |16,

secondary winding of transformer |10 and a portion of resistor |13 to the input of a filter |10 having a constant resistance R which equals 15,000 ohms at all the frequencies of the current impressed upon it from vthe 60cycle source fiow-I ing through the anode c ircuit of the gas-filled tube. The filter |16 is a low-pass filter cutting off at about 42 cycle-. It comprises the elements labeled L,V2L, C, 2C and R, the value of L being 40 henries, vthat of C being .175 microfarad and The output of filter |14 is connected to the filter comprising the series inductive element .|15 of 40 henries and the. shunt circuit comprising the inductive element |16 of 1 henry and the condenser |11 of 2 .microfarads in series, the outputresistor. |56 of 15,000 ohms being connected across the shunt circuit comprising the elements |16 and |11. The circuit comprising elements |15, |16 and |11 is a low-pass filter having a cut-off frequency somewhat below cycles.

The filter current fiowing through the resistor varied in accordance with the average tone value of the field of view being scanned under control of the correcting impulses of the television signal. The condenser |55 is charged to a voltage Across the secondary winding of transformer .the charged condenser is connected to the cathode of the cathode ray tube |36. Therefore, as the voltage to which the condenser is charged increases, the bias of the control electrode of the cathode ray tube becomes more positive or less negative.-

The correcting impulses of the signaling eleciromotive force impressed upon the control electrodes of vacuum tubes |34 and |64 are of negative polarity. The correcting impulses impressed upon the control electrode of tube |35, therefore, will be of positive polarity and the correcting impulses impressed upon the control electrode or grid of the cathode ray tube |36 will be of negative polarity and willv be reproduced as black up'on the cathode ray tube screen. The polarity of the correcting impulses of the television signal impressed upon the grid of the gas-filled tube |65 will be positive. These impulses cause current to flow in the anode circuit of the gas-lled'tube and through resistor |56 to cause the condenser |55 to be charged.

The operation of the corrector circuit of Fig. 1 maybe understood from the diagram of Fig. 9. The dotted line X shows the static characteristic of the gas-nlled tube |65. Any combination of anode voltage EP and grid voltage Eo. lying on the line X or above this line, as viewed in this figure, will cause space current-to flow in the anode circuit of tube |65. The ellipse Y shows the dynamic characteristic of the tube |65 when no signals are being received from the transmitter. In the diagram the plate voltage above zero is positive and the grid voltage to the left of zero is negative. As the sinusoidal (iO-cycle voltage applied to the grid becomes less negative and the (iO-cycle voltage applied to the anode becomes'less positive (going along the ellipse in the direction indicated by the arrow), the ellipse will just touch the line X at point A and the tube will commence to pass space current at this point'duringv each cycle of the 60-cycle wave. Current is thus passed to the filter |14 until the plate voltage falls to a certain value below which no current flows in the anode circuit. No current will flow during the remainder of the cycle until the point A is again reached. When signals are received from the transmitter T, impulses the amplitude of which measured between the peak amplitude -and zero is representative' of the average tone value of the image, are applied to the control electrode of tube |65 in addition to the 60-cycle voltage. If, for example, the image field is scanned 2O times per second along 240 lines and a correcting impulse is produced once per scanning line, there will be produced 4,800 correcting impulses per second or 80 impulses during one cycle of the (S0-cycle current. When the average tone value of the field of view `being scanned changes from dark to light, the amplitude of the correcting impulses increasesv and the tube |65 commences to pass space current earlier in the cycle, for example, at point B. Therefore, anode current will iiow over a longer time during each cycle and the average anode `current flowing through resistor |56 will be correspondingly inc reased. Thus, the bias on the cathode ray tube grid will be more positive due to the higher potentlal to which the condenser |55 is charged and the image produced will have'a lighter average tone value corresponding to the average tone value of the eld of view being scanned. 'I'he voltage drop due to the flow -of anode current through the resistive portion of the anode-cathode circuit of tube |65 reduces the voltage across the anode-cathode circuit by an amount which increases as the anode current increases. The efect of an increase in anode current in tube |65. therefore, is to increase the output voltage across the resistive element |56 (Emmi) and to decrease the anode-cathode voltage (Ep). That is, the ellipse of Fig. 9 is displaced downward, the center of the ellipse being moved in a negative direction along a line parallel to the ordinate EP. Thus, when the correcting impulses increase to a certainamplitude, the output voltage to which condenser is charged increases correspondingly. However, the output voltage is increased only by a certain amount, depending upon the increase in amplitude of the correcting impulses,v because an increase in the output voltage causes va decrease in the anode voltage and due to this decrease in anode voltage the correcting impulses will no longer be of s'licient amplitude to start the flow of space current at point B. As long as the amplitude of the correcting impulses remains constant, suilicient current will flow in the anode circuit of tube to maintain the voltage across the condenser |55 at a constant value. When there is a decrease in amplitude of the correcting impulses, the condenser |55 will partially discharge through the portion of resistor |56 which is connected across the condenser.

Fig. 3 shows an arrangement which may be substituted for the portion of the apparatus shown to the right of lines X-X and Y-Y of Fig. 1. In this figure, incoming television image current after being transformed by the trans former |00 and amplified by ampli'iier |80 is applied to the input circuit of the thermionic tube |8| by means ol a coupling condenser |82, the grid |83 of the tube |8| being biased by biasing battery |84 to which the negative terminal of the Ybattery is connected through a resistor |85. The

output circuit of the tubel |8| is coupled by resistor |86 and condenser |81 to the input circuit of the amplifier tube |88 through resistor |89. 'I'he output circuit of the tube |88 contains the l neon lamp 38, thyrite resistor 39, ballast resistor' 19 and source of plate potential 40. 'I'he potential drop across the resistor |89 produced by the incoming correcting impulses, controls apparatus for varying the average current in the neon tube circuit in a manner which will now be described.

A transformer primary coil |90 is coupled to secondary coils |9| and |92, which primary coil |90 receives 60-cycle alternating current from the power source |93 through the [variable capacitor |94. The secondary coil |9| is connected in the plate circuit of a gas-filled electron discharge device |99. Coil |92 isvconnected in circuit with a copper-oxide rectifier |95 and-a lter vcircuit including condensers |96 and |91 and inductor |98, which filter-circuit is connected across the resistor |69.v Sixty-cycle currentirom the source |93 is also impressed upon the grid-cathode circuit of the gas-filled device |94 through the transformer 200,- the 60-cycle electromotive force impressed upon the grid circuit being substantially degrees out of phase with respect to I that impressed upon the plate circuit. In the input circuit of the gas-filled electron discharge device |99 are connectedin series an uncoupling tube 260.

resistor of high resistance to prevent any large current from flowing in grid-cathode circuit of tube |99, biasing battery 202, a tapped portion of the resistor |85 and battery |84.

'I'he operation of the gas-filled device |99 will now be described. As transformer coil |90 is coupled to both secondary coils |9| and |92, with no incoming correcting impulses, the coil |9| has a light load and the rectifier |95 delivers maximum voltage to the filter represented by the numbers |96, |91 and |98 and current of relatively large amplitude flows through the resistor |89. This makes the cathode of the'tube |88 positive with respect to its control electrode, thus causing a decrease in the average space current through neon lamp 38. When signals come in to the gas-filled discharge device |94 itlpasses more space current. As a result, increased current flows through the winding |90 and condenser |94, the voltage drop across the condenser increases and the voltage delivered by the coil |92 and the rectifier |95 is cut down. Due to the resulting decrease in current through resistor |89, the space current through tube |88 increases.

Fig. 4 shows a modied arrangement for producing a direct current component of a television image current corresponding to the average e image tone value under control of a television put of the rst circuit serves to control the output voltage of a high' voltage rectier which constitutes the secondpart of the circuit. In this circuit the television image signal after being transformed by the transformer |00 and amplified by the amplifier 2|0 is applied by means of the transformer 2|| to the input circuit of an electron discharge device 2|2, the output circuit of which contains the primary of a transformer 2|3. The secondary of the transformer 2|3 is connected through a potentiometer resistor 2|4, the mid-point of which is connected through resistor 2|5 to the grid 2|6 of a gas-filled electron discharge device 2|1 and through resistance 2|8 to the grid 2|9 of an electron gas-filled,- discharge device 220. 'I'he device 2|2 is an isolating stage and serves to prevent kicks from gas-filled tubes 2|1 and 220 from getting into the signal In some cases this tube 2|2 may be eliminated. Resistors 2|5 and 2|8 are isolating resistors and they prevent the tube 2|1 from firing tube 220 and vice versa. Sixty-cycle alternating current is applied to the plate circuits of the gas-lled tubes 2|1 and 220 by means of a transformer 22| having split secondaries 222 and 223. A resistor 224 limits the anode currents for the tubes 2|1 and 220 to-a safe value, and condenser 225 and choke coil 226 smooth the impulses in the plate circuit before the current reaches the bias winding 221 and 228 of the impulse coils 229'and 230, respectively. By means of transformer 23|, 60-cycle current from the source 232 is applied to the resistors 233 and 234 through the phase shifting condenser 235. The grid of each gas-filled tube 2|1 and 220 is thus driven by 60-cycle voltage in additionto any signals from the tube 2|2.

The static characteristic of the tube 220 o1 2|1 is shown in Fig. 6, the abscissa Ea being the voltage applied to the grid with respect to the cathode and the ordinate Ep being the voltage applied to the anode with respect to the cathode. Any combination of grid and plate voltage to the left of the line A will not causecurrent to pass, but any combination of grid andA plate voltage which .falls on, or to the right of, the line A will cause the tube to re and it will continue to pass current until the plate voltage EP is reduced below the ionization potential.

{I'he periodic reduction in the voltage EP below the .ionization potential is produced by impressing E50-cycle alternating voltage EP plus a direct current voltage En upon theplate circuit of the gasiilled tube. The time relations are shown in Fig. 5, in which thecycle variations of EP and EG are plotted against time. sents the firing line. This line represents the valueof grid voltage required to discharge the tube, this value fluctuating with the cyclic variations of Ep. If at any given time the grid voltage is less negative than the voltage represented byl the dotted line, the plate voltage being positive, the tube will pass current until Ep falls below the ionizing potential of the particular gas used in the tube. When no signals are incoming the grid voltage line'rst meets the firing line at point A. This is late in the cycle and but little current is passed. When signals are superposed on Eo (as shown, for example, in the left-hand half cycle in Fig. 5) the tube breaks down earlier. The breakdown point is represented by point B. The higher the signal peaks the earlier the breakdown will occur in the alternating current cycle and the greater the output current. The signals are so poled that the black" correcting pulses cause` discharges in the gas-lled tubes. The diierence in voltage at the firing point between the curve representing the black signals superposed upon the grid voltage wave and the dotted curve must be less than at any point earlier in the alternating current cycle. If the adjustments are not made so that this is true, an impulse may fire the tube at the beginning of the cycle. Assuming the adjustments are correct, as

the black peak becomes higher, ring occursA earlier in the cycle. The average output current of the gas-filled tube thus increases and the average plate potential decreases, due to the drop through the output circuit.

The rectifier part of the circuit will now be described. This rectiiier supplies the voltage to the capillary or neon lamp 38. By controlling the rectifier te'rmlnal voltage, the average current through the lamp is changed. The biasing windings 221` and 228 of the impulse transformers 229 and 230 are connected in a circuit which goes from the negative terminal of battery 236 through the opposing resistors 233 and 234 to the cathodes of gas-filled tubes 2|1 and 220, through the discharge paths of these tubes and through secondary windings 222 and 223, resistor 224, choke winding 226, coils 221 and 228 and thence to the positive terminal of battery 236.

Coils 231 and 238 are supplied with SO-cycle current from the generator 232 through variable resistor 239.. Transformer 240 supplies voltage from generator 232 through the phase shifting condenser 24| to the resistors 242 and 243 portions of which are connected inthe grid circuits of mercury vapor rectiiier tubes 244 and 245, the

The dotted graph represecondary coils 248 and 241 of the impulse transformers229 and 230 also being connected in this circuit. A battery 248 supplies xed bias for the grids of these two mercury vapor devices. Coils 221, 231 and 248 are thus the windings of an impulse transformer which ilres tube 244, and coils 228, 238 and 241 are the coils of the impulse transformer 230 which fires tube 285. These impulse transformers have cores which are constricted in cross-section where the secondary windings 246 and 241 are located. This section of the core saturates sharply and for this reason the secondary only delivers voltage for a short part of the alternating current cycle. For a more complete description of the impulse transformer per se reference should be made to the United States Patent 1,880,412 to Burton, October 4, 1932.

Sixty-cycle alternating current is also applied to the output circuits of the mercury vapor tubes 244 and 245 by means of a transformer 248 having split secondaries 250 and 215|. Transformers 252 and 253 may be used to supply heating current to the cathodes of the tubes 244 and 245. The outer terminals of secondary windings ,250 and are connected to the anodes of tubes 244 and 245. The mid-point o the secondaries of the transformers 252 and 253 and the common terminal of the secondary windings 250 and 25| are connected through a filter circuit comprising nductances 254 and 255 and condensers 258 and 251 to the load circuit which includes the neon lamp 38p thyrite resistor 39, and ballast resistor 19. Also connected in this load circuit is the output circuit of the electron discharge device 280, the input of which is fed by the secondary 26| oi' the transformer 2li. The tube 280 is adapted to impress upon the lamp 38 the alternating current components of the received image current. 'I'he lamp 38 and the thyrite resistor 39 are connected to the lead going to the positive terminal of the rectifier so that the capacity of the rectifier and iilter circuit to ground will not act as a shunt across the lamp 38 and the thyrite resistor 39 at the higher frequencies of the television signal. The output oi' the rectiers 244 and 245 serves to insert direct current component into the current through the neon tube 33.

The time relation between the grid and plate voltage for one of the rectifier tubes is shown in Fig. 8. Ep is the plate voltage, Eo the grid voltage and the dotted line is the breakdown or firing curve. Although but one dotted line is shown, there are an infinite number of them and the temperature of the tube determines which relation exists at any one time. For instance when Ec=6 volts, Ep=700 volts for ring at 20 C., but if the bulb warms up to 35 C., Ep will fall to 540 volts for the same grid voltage. 'Ihls means a change of 160 volts in the output voltage. By using a sharp impulse to fire the tube as. for example, those induced in the secondary windings 248 and 241 of the impulse transformers 229 and 238, the eilect of temperature is made negligible because the sharp impulse pierces all the breakdown curves at once. It is for this. reason the signal impulses are not used directly to fire the rectifier tubes, but are used to produce a direct current proportional to the direct current component at the transmitter and this is impressed upon impulse transformers instead.

-As the corrector circuit increases the bias current in the coils 221 and 228 of the impulse transformers 229 and 230, pulse A through the seccycle and this causes the tube to fire earlier in the alternating current cycle which in turn causes the output voltage to rise. Pulse B has no eilect. When the corrector circuit decreases the bias current in the coils 221 and 228, the opposite eiiect is produced.

Fig. 7 shows the kcurrent-time relations of the impulse coil exciting current Is, the bias current In, and the secondary impulses Is produced in the coils 248 and 241. The nicks D and E in the In wave are caused by an increase of impedance in the secondary coils 248 and 241, when the cores unsaturate. A graph similar to Figs. 'T and 8 for the other rectifier tube would merely show EP shifted 180 degrees together with its Eo, In: and Is.

In Figs. 3 and 4 ythe neon lamp 38 and scanning disc 54 (like the scanning disc 2|) are employed for producing the television images instead of a cathode ray tube as shown in Fig. 1. Any suitable system for maintaining the discs 2| and 54 in synchronism may be used. In one such system, which is herein described by way of example, alternating current of substantially constant frequency is produced by a vacuum tube oscillator 51 which is located at the transmitting station for controlling the operation of the motor speed control circuit 53 at that station and current from this source is also transmitted over line 59 to the receiving station R for controlling the operation of speed control means 50 at that station. The motors 23 and 58 at both stations are thus maintained in synchronism. Suitable amplifiers 6| and 82 at the transmitting and receiving stations, respectively, are used to amplify the generated current. For a more Vcomplete description of a synchronizing system herein briefly described, reference may be made to United States Patent 1,999,376 of H. M. Stoller, issued April 30, 1935.

An observer E may view, the image produced by the cooperation of the receiver scanning means and the light source 38 through an opening 83 in an opaque screen 84. Glow lamp 38 may be of any desired type adapted to supply light which varies with the variations in amplitude of the applied image current. A suitable lamp for this purpose is disclosed in United States Patent 1,918,309 of H. W. Weinhart, issued July 8, 1933. As therein described, the glow lamp may include a charge comprising a rare gas and a small percentage of active gas, as disclosed in United States Patent 1,871,266 of F. Gray, issued August 9, 1932.

It apparent that the action of the correcting circuits disclosed herein is such that the periodic impulses as they appear in the control electrodecathode circuit of cathode ray tube |35 (Fig. 1) or in the circuit of lamp 38 (Figs. 3 and 4) are of constant amplitude.v For example, if in Fig. 1 the correcting impulses of negative polarity supplied to the cathode ray tube from the amplifier |35 increase in amplitude, the voltage across condenser |55 also increases sumciently to maintain the amplitude of the impulses impressed upon the control electrode-cathode circuit at con-v stant amplitude.

What is claimed is:

1. In a system including a source of television image electromotive force which is devoid of the direct component representative of average tone value of a eld of view and having recurring impulses of predominant amplitude, the amplitude -of which varies in accordance with the variations of said direct component, the combination of an electric discharge device having a control circuit under control of said television electromotive thereto, electrically associated with the output circuit of said gas-lled tube, means for impressing said television electromotive force upon the control circuit of saidgas-lled tube to control the amplitude of the unidirectional current iiowing in said biasing circuit in accordance with the amplitude of the recurring impulses of the televisionimage electromotive force, and means for connecting said biasing circuit to the control circuit of said electric discharge device to control 4the biasing thereof.

2. In combination, a gas-lled electric discharge device having a cathode, an anode, a control electrode, an anode-cathode circuit and a control electrode-cathode circuit, a source of alternating current in said anode-cathode circuit, a second source of alternating current of the same frequency as that of the first source but out ofphase with respectthereto in said control electrode-cathode circuit, and means for impressing periodically recurring voltage impulses of higher frequency and of varying amplitude upon said control electrode-cathode circuit for controlling the average amplitude of the current flowing in said anode-cathode circuit in accordance with the amplitude of said voltage impulses, said impulses causing anode current -to ow in said discharge device during a portion of each cycle of said alternating current commencing at a time controlled by the amplitude of the impulses but always during the portion of the cycle in which the anode-cathode voltage is decreas-= ing and the control electrode-cathode voltage is increasing. t

3. In combination, a gas-lled electric discharge device having a cathode, an anode, a control electrode, an anode-cathode circuit and a control electrode-cathode circuit, a portion of said anode-cathode circuit being in said control electrode-cathode circuit, a source of alternating current in said anode-cathode circuit, asecond quency as that of the first source but out of phase with respect thereto being in the common portion of saidcircuits, means for impressing periodically recurring voltage impulses of varying amplitude and oi brief duration with respect to the intervals between successive impulses upon said control electrode-cathode circuit for controlling the average amplitude of the current iiowing in said anode-cathode circuit in accordance with the amplitude of said voltage impulses, the anode potential of said device decreasing while the control electrode potential is increasing and said impulses further increasing the potential of said control electrodeto cause anode current to flow during a portion of each cycle of said alternating current and a constant resistance filter connected to said anode-cathode circuit for suppressing, in part at least, alternating components of the anode-cathode current.

4. In combination, a gas-lled electrical' discharge device having a cathode, an anode, a control electrode, an anode-cathode circuit and a control electrode-cathode circuit, a portion of said anode-cathode circuit being in said control electrode-cathode circuit, a source of alternating current in said anode-cathode circuit, a second source of alternating current of the same frequency as that of the rst source but out oi phase with respect thereto being in the common portion of said circuits', means for impressingperiodically recurringvoltage impulses upon said control electrode-cathode circuit for controlling the average amplitude of the current owing in said anode-cathode circuit in accordance with the amplitude of said voltage impulses, a constant resistance filter connected to said anode-cathode circuit for suppressing, in part at least, alternating components of the anode-cathode current, a cathode ray image producing device having a control circuit, a terminating resistor for said iilter, a' condenser connected to said terminating resistor, said condenser being charged to a potential corresponding to the slowly vary- Vsource of alternating current of the same freing unidirectional current flowing through said terminating resistor, and means for connecting said condenser to said control circuit for supplying a` biasing potential thereto.

5. In a television system, a, source of television image electromotive force which is devoid of the v slowly changing unidirectional component representative of average tone value of a eld of view and having periodic impulses the amplitude of which is representative of the average tone value of the iield of view, an electric discharge device having an input and an output circuit. a. source of alternating electromotive force having a frequency lower than the frequency of recurrence of said periodic impulses, means yfor impressing alternating electromotive force from said source upon said input and said output circuits and means for impressing an electromotive force from said source of television electromotive force upon said input circuit to produce insaid output circuit an electromotive force corresponding to said slowly. changing unidirectional component representative of average tone value. 

